5.5.1 Automated signature checking

It is very important to understand the semantics used with signature
verification. Checking a signature is not as simple as it may sound and
so the operation is a bit complicated. In most cases it is required
to look at several status lines. Here is a table of all cases a signed
message may have:

The signature is valid

This does mean that the signature has been successfully verified, the
certificates are all sane. However there are two subcases with
important information: One of the certificates may have expired or a
signature of a message itself as expired. It is a sound practise to
consider such a signature still as valid but additional information
should be displayed. Depending on the subcase gpgsm will issue
these status codes:

signature valid and nothing did expire

GOODSIG, VALIDSIG, TRUST_FULLY

signature valid but at least one certificate has expired

EXPKEYSIG, VALIDSIG, TRUST_FULLY

signature valid but expired

EXPSIG, VALIDSIG, TRUST_FULLY
Note, that this case is currently not implemented.

The signature is invalid

This means that the signature verification failed (this is an indication
of a transfer error, a program error or tampering with the message).
gpgsm issues one of these status codes sequences:

BADSIG

GOODSIG, VALIDSIGTRUST_NEVER

Error verifying a signature

For some reason the signature could not be verified, i.e. it cannot be
decided whether the signature is valid or invalid. A common reason for
this is a missing certificate.